JPH068483B2 - Non-aging bake hardenable cold rolled steel sheet with excellent local ductility manufactured by continuous annealing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to IF steel (Interstitial Atoms Free Steel, hereinafter referred to as IF steel) (carbon in steelmaking is lowered to an extremely low level of several ppm, and carbon is further reduced). Ti and N that have a strong affinity with
Non-aging and local ductility similar to box-annealed materials by using low carbon A-killed steel with normal carbon level, regardless of the solid solution carbon and nitrogen completely removed from the steel by adding b). It also relates to JIS G3141 Class 3 deep-drawing cold-rolled steel sheet "symbol SPCE" that has both bakeability and bake hardenability (hereinafter referred to as BH property).

(Prior Art) The production of SPCE grade by continuous annealing was mainly made of IF steel. This IF steel is based on the invention described in JP-B-44-18066, and many improvements and improvements have recently been made. However, IF steel requires a long time for vacuum degassing in steel making, and is economically inferior to the conventional box-annealed A-killed steel, and thus steelmaking facilities also need to be greatly restricted. Further, it becomes extremely difficult to impart BH property.
Continuous annealing SPCE with normal A-killed steel regardless of IF steel
Attempts have also been made to produce grades, but they have not been used because of the two basic problems of poor strain aging resistance and local ductility.

(Problems to be Solved by the Invention) With respect to strain aging, in recent years, many inventions aimed at suppressing solid solution carbon and suppressing aging by optimizing cooling / overaging treatment of continuous annealing have been proposed. Has been done. In order to prevent stretcher strain that impairs the appearance quality of press-formed products, the aging index (hereinafter referred to as AI) is 3 kgf /
It is generally well known that the thickness needs to be less than mm ² . On the other hand, in order to sufficiently precipitate the solid solution carbon that causes the strain aging by a short-time overaging treatment such as continuous annealing, it is advantageous to distribute a large amount of carbide serving as a precipitation site in the grains. However, Takahashi et al. (The Metallurgical Society of
AIME, 1982, p51), Hosoya et al. (The Metallurgical Societ
y of AIME, 1984, p61) and Japanese Examined Patent Publication No. 59-59832, it is clarified that increasing the intra-grain carbide density increases the yield strength (YP) and decreases the ductility. Therefore, a cold-rolled steel sheet that is non-aging and excellent in workability needs to have an appropriate amount of solute carbon and an intragranular carbide density in reality.

Solid solution carbon, which causes strain aging, also has the advantage of hardening the material by baking coating after press molding and improving the dent resistance of the panel, so by leaving an appropriate amount of solid solution carbon It is most desirable that a high BH property is ensured without the occurrence of letcher strain. However, the conventional IF steel or the A-killed steel obtained by box annealing has almost no residual solid solution carbon, so that it is extremely non-aging, but it is extremely difficult to impart BH property.

Regarding local ductility, its evaluation index is not clear so far, and it cannot be evaluated by ductility in a normal tensile test,
Moreover, the material factors that influence the local ductility were not clear.

However, based on the results of the research conducted by the present inventors, A. K. LDH ₀ (Met. Eng. Quart., 15 (1975) 53) proposed by GHOSH was found to be effective as described later (although the test method is almost in accordance with the proposal of AK GHOSH. For details, see Example 1.). It has also been clarified that the most basic technique is to optimize the balance between the intragranular carbide density and the amount of solute carbon in order to achieve both the non-aging property and the processing method and to secure the BH property. . Therefore, according to the present invention, by controlling the intragranular carbide density and the amount of solute carbon, both the non-aging property and the BH property can be achieved, and the local ductility of the A-killed steel sheet by the conventional continuous annealing is inferior, which is a problem for deep drawing. A cold rolled steel sheet is provided.

(Means for Solving the Problems) The gist of the present invention is C: 0.008 to 0.025% by weight% and Mn:
0.05 to 0.20%, S: 0.004 to 0.015%, acid soluble A: 0.03
~ 0.15%, 0.012% or less of P as an impurity, N
Of 0.0030% or less and the balance Fe and unavoidable impurities are steel sheets (strips) obtained by performing ordinary hot rolling, followed by cold rolling-continuous annealing-temper rolling, and forming a solid solution. The carbon content is 5 ppm or less and 2 ppm or more, and the intra-grain carbide density (ρ,
Co / mm ² ) is defined as the range of the following equation which is a function of the aging index (AI, kgf / mm ² ), and ρ ≦ (−0.1941 × AI + 3.8831) × 10 ⁵ , and ρ ≦ (−0.6415 × AI + 4.8048) ) × 10 ⁵ Yield strength (YP) after aging treatment (100 ℃ × 1h) is 19kgf /
It is a cold rolled steel sheet having a bake hardenability of ^{2 to 5} kgf / mm ² and having an LDH ₀ of 30.9 mm or more, which is an index of local ductility, of mm ² or less.

(Function) First, the reason why the numerical values of the C, Mn, P, S, A, and N components of the present invention are limited will be described.

The lower limit of C is limited from the viewpoint of aging. That is, in overaging precipitation, it is necessary to increase the degree of supersaturation of C in order to increase the driving force for precipitation, but if the content of C is small, the degree of supersaturation of C does not increase even if quenching is performed after annealing. In this sense, C needs to be 0.008% or more. The upper limit of C is limited by the workability, and if 0.025% or more, the desired workability cannot be obtained. The lower limit of Mn and the upper limit of S are limited from the viewpoint of preventing FeS generation. That is, when the Mn / S ratio is small, S becomes FeS and precipitates, which causes hot brittleness. Therefore, Mn must be 0.05% or more and S must be 0.015% or less. On the other hand, the lower limit of S is limited to secure an appropriate MnS. That is, MnS is a precipitation site of cementite that precipitates in the grains, and by appropriately dispersing MnS, the nucleation density of cementite is appropriately increased, the diffusion distance of carbon is shortened, the time required for diffusion is shortened, and non-aging occurs. Promote the transformation. Therefore, S is 0.004% or more. The upper limit of Mn should be 0.15% or less from the viewpoint of workability.
A must be 0.03% or more in order to promote AN precipitation, but if it is too much, it hardens the steel and deteriorates the r value, so it must be 0.15% or less. P and N
Should be 0.012% and 0.0030% or less, respectively, from the viewpoint of workability.

Next, in order to investigate the relationship between the amount of solute carbon and the BH property, the BH properties of materials having various amounts of solute carbon were investigated. The results are shown in FIG. From the perspective of stretcher strain A
I ≦ 3 kgf / mm ² is required, but for this purpose, the amount of residual solid solution carbon must be 5 ppm or less. On the other hand, the lower limit is limited by BH, and in order to give BH of ² kgf / mm ² or more, the amount of residual solid solution carbon must be 2 ppm or more. It can be seen from FIG. 1 that it is possible to apply a maximum of 5 kgf / mm ² of BH while satisfying the above conditions.

The local ductility problem in the continuous annealed material of low carbon A-killed steel, which has been unclear in the past, is a plane strain fracture that occurs in an actual press site as a result of detailed examination from various viewpoints, and this is a major cause of fracture. There was found. Therefore, the local ductility means the ductility in the plane strain state, and in the plane strain state, the diffusion necking and the local necking occur almost at the same time, which is the most severe deformation state for the material. But,
Ductility under plane strain deformation cannot be evaluated because a plane strain state cannot be obtained by an ordinary tensile test. As an evaluation method of ductility under such plane strain deformation, A. K. Focusing on LDH ₀ proposed by GHOSH, it was found to be the most simple and effective. Therefore, in the present invention, LDH ₀ is adopted as an evaluation index of local ductility. Next, the controlling factors of LDH ₀ were investigated. The results are shown in FIGS. Figure 2 shows the relationship between AI and solid solution carbon and LDH ₀ by aging treatment (100 ° C x 1
h) It is an examination before and after. FIG. 3 shows the relationship between LDH ₀ and the carbide density of a material in which the solid solution carbon content is 0 ppm and the carbide density is continuously changed. The solid solution carbon amount was determined by the internal friction method, and the carbide density was determined by
It was determined by extraction replica electron micrograph at 10000 times. From these results, it was found that LDH ₀ was significantly deteriorated due to strain aging and increase in intra-grain carbide density.

On the other hand, in order to sufficiently precipitate solid solution carbon by a short-term overaging treatment such as continuous annealing, it is advantageous to distribute a large amount of carbide serving as a precipitation site in the grains. However, if the intra-grain carbide density is excessively high, the yield point (YP) rises and L
DH ₀ decreases. If the precipitation of solute carbon is insufficient, strain aging increases YP and decreases LDH ₀ . Therefore, it is necessary to control the amount of solute carbon and the density of intra-grain carbide appropriately. Therefore, in order to clarify this condition, materials having various amounts of solute carbon and intragranular carbide density were prepared by heat treatment as shown in FIG. In the heat treatment (A) of FIG. 4, the intragrain carbide density increases as the quenching end temperature (TE) decreases, and the solid solution carbon amount decreases as the overaging time (tOA) increases. In addition, in FIG.
As a result, the intragranular carbide density increases, and as the tOA increases, the amount of solute carbon decreases. For the samples thus obtained, immediately after temper rolling and after aging treatment (10
LDH ₀ test and tensile test were performed at 0 ° C x 1h)
The results shown in the figure were obtained. Fig. 5 shows the amount of solute carbon and the intra-grain carbide density of the test material after aging treatment (100 ° C x 1
3 shows LDH ₀ and YP of h). As a result, if the following conditions are satisfied, YP ≦ 19 kgf / mm even after aging treatment.
² (upper limit of A-killed steel by box annealing), LDH ₀ ≧ 30.
It was found to satisfy 9 mm (average of A-killed steel by box annealing).

Aging Index (AI) from the perspective of stretcher strain
Of 3 kgf / mm ² or less, and in terms of shape fixability (YP, kgf / mm ² ) and local ductility (LDH ₀ , mm), intragranular carbide density (ρ, co / mm ² ) ρ ≤ (-0.1941 x It is controlled to AI + 3.8831) × 10 ⁵ (1) ρ ≦ (−0.6415 × AI + 4.8048) × 10 ⁵ (2).

In the conventional A-killed steel by continuous annealing, such a balance between the amount of solute carbon and the intra-grain carbide density has not been optimized, so that the above range of ρ has not been used. . Accordingly, the present invention is a cold-rolled steel sheet for A-killed deep-drawing by continuous annealing, which has been considered impossible to manufacture conventionally, by optimizing the balance between the amount of solute carbon and the intra-grain carbide density, and further IF steel or box annealing. It is a cold-rolled steel sheet having BH property, which is difficult to impart with A-killed steel.

Next, how the present invention is useful as compared with box annealed A-killed steel and IF steel will be described with reference to examples.

(Example-1) A steel having the chemical composition shown in Table 1 was tapped, and a slab was manufactured by a continuous casting method.
It is heated to ℃, hot-rolled so that the finishing temperature is 860 to 880 ℃, and the plate thickness is 4.0 mm. Then, it is cooled on the runout table so that the average cooling rate is 15 ℃ / s, and then
It was wound into a coil at 690 ° C. After pickling this hot rolled steel strip 0.8
The steel sheet was cold-rolled to a thickness of mm to form a cold-rolled steel strip, and subsequently, this cold-rolled steel strip was subjected to continuous annealing. The conditions for continuous annealing are: annealing temperature: 82
0 ℃, soaking time: 1min, first slow cooling: 690 ℃ up to 5.5 ℃ /
Cooling at s, rapid cooling thereafter: cooling to 120 ° C / s up to 250 ° C, supercooling condition: holding at 250 ° C for 2s, reheating condition: heating to 340 ° C at 30 ° C / s, overaging condition, end temperature The overaging time was 270 ° C and 200s. After that, skin pass rolling was performed at an elongation rate of about 1.0% and the test was performed.

As a test, a tensile test and an LDH ₀ test were performed immediately after skin pass rolling and after artificial aging treatment at 100 ° C. × 1 h. The tensile test was performed using JIS Z2201, No. 5 test piece according to the method described in the same Z2241. Further, the AI and BH properties, the amount of solute carbon, and the intra-grain carbide density were also determined. AI is 100% after 60% pre-strain x 60 min
Artificial acceleration aging is performed and the increase in yield strength before and after this aging is shown. BH is shown by the amount of increase in yield strength before and after this artificial aging at 170 ° C for 20 min after 2% prestrain.
The amount of solute carbon was determined by the internal friction method, and the intra-grain carbide density was 1000.
It was determined by a 0x extracted replica electron micrograph. L
In the DH ₀ test, a 100 mmφ ball head punch and a 108 mmφ die were used, the punch speed was 40 mm / min, it was confirmed that there was no material inflow, the wrinkle holding pressure (BHF) was 30 tons, and the actual press conditions were taken into consideration. Rust oil lubrication was used.

The test results are shown in Table 2. As is clear from this table, the steels A, B, and C according to the present invention have the formability similar to that of a box annealed material and have a BH of ^{2 to 5} kgf / mm ² . On the other hand, Steel D is not suitable from the viewpoint of stretcher strain because AI = 4.5 kgf / mm ² , and further, YP remarkably increases after aging treatment due to strain aging, and LDH ₀ also remarkably decreases. Steel E has a high YP and a low LDH ₀ immediately after the skin pass because of the high intra-grain carbide density. Further, Steel F is high in both AI and intragranular carbide density and is not suitable in all aspects.

(Example-2) Using steel B in Table 1, continuous casting-hot rolling-pickling-cold rolling was performed under the same production conditions as in Example-1, and continuous annealing was performed under the conditions shown in Table 3. went. After that, skin pass rolling was performed at an elongation rate of about 1.0% and the test was performed. The test method is the same as in Example-1.

The test results are shown in Table 4. The heat treatment code in Table 4 corresponds to that in Table 3. Steel a in Table 4 has AI = 4 kgf /
Since mm ² and b are AI = 5 kgf / mm ² , they are unsuitable from the viewpoint of stretcher strain, and YP remarkably increases after aging treatment due to strain aging, and LDH ₀ also remarkably decreases. Steels c and d have a low AI = 1, 0.5 kgf / mm ² and there is no problem that the formability is deteriorated due to strain aging or stretcher strain is generated, but since the intragranular carbide density is high, it is already after the skin pass. YP is high and LDH ₀ is low. further,
It has almost no bake hardening ability. In contrast to these steels, the steels e, f, and g of the present invention show the formability similar to box annealed materials, and
It has a BH property of ˜5 kgf / mm ² .

(Effect of the invention) Cold-rolled steel sheets are consumed in large quantities in automobiles, electric appliances and building materials,
It is also a material for surface-treated steel sheets. The present invention relates to a cold-rolled steel sheet used for such an application, and SP by continuous annealing having unprecedented good formability and BH property.
It is a CE grade low carbon A kill cold rolled steel sheet. Therefore, an extremely large effect is expected in the industry.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings show the technical contents of the present invention, and FIG.
The chart which shows the relationship of I, BH, and YP-E. Figure 2 shows L
Chart showing the effect of dissolved carbon content on the DH _0. FIG. 3 is a chart showing the effect of intragranular carbide density on LDH ₀ . Figures 4 (A) and (B) show the heat treatment cycle of the test material. Fifth
The figure shows AI, carbide density in grain and aging treatment (100 ℃ × 1h)
Is a table showing the _LDH 0, the relationship of YP and BH after.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Akisue 5-10-1 Fuchinobe, Sagamihara City, Kanagawa Pref., Second Research Laboratory, Nippon Steel Corporation (56) Reference JP-A-1-177320 (JP, A) JP-A-61-276935 (JP, A)

Claims (1)

[Claims] 1. In weight%, C: 0.008 to 0.025%, Mn: 0.05
~ 0.20%, S: 0.004 to 0.015%, acid-soluble A: 0.03 to 0.
It contains 15%, 0.012% or less of P and 0.0 of N as impurities.
A steel sheet containing 030% or less and the balance Fe and unavoidable impurities, which is obtained by performing normal hot rolling, followed by cold rolling-continuous annealing-temper rolling, wherein the steel sheet is a solid solution. Carbon content 5ppm or less 2ppm or more, grain carbide density (ρ, co / mm
² ) is within the range of the following formula, which is a function of the aging index (AI, kgf / mm ² ), ρ ≦ (−0.1941 × AI + 3.8831) × 10 ⁵ , and ρ ≦ (−0.6415 × AI + 4.8048) × 10 ⁵ yield strength after aging treatment (100 ℃ × 1h) (YP ) is 19kgf
/ mm ² or less, LDH ₀ which is an index of local ductility is 30.9 mm or more, and has a bake hardening ability of ^{2 to 5} kgf / mm ² , which is excellent in local ductility produced by continuous annealing. Hardenable cold rolled steel sheet.